Meson fixes for gtksink on Linux, and fix buffering when logging in cargo_wrapper

cargo_wrapper: Write to log with line-buffering

So we get log output while cargo is running, not just when it completes

meson: Enable gstreamer-gl-1.0 features in gtk4 plugin

Basically, if gstreamer-gl-1.0 is built with wayland / x11 / egl, use
those features in the gtk4 plugin.

MacOS always uses CGL, and it's always available. Windows version does
not use GL yet.

Requires gstreamer!3654 (merged)